Nanoparticles exhibit intriguing changes in electronic, optical, and magnetic properties as a result of the nanoscale dimensionality (Daniel et al., Chem. Rev. 104:293 (2004); Xia et al., Adv. Mater. 12:693 (2000)). The ability to engineer size and monodispersity is essential for the exploration of these properties. The preparation of magnetic nanoparticles and nanocomposites has attracted both fundamental and practical interest because of potential applications in areas such as ferrofluids, medical imaging, drug targeting and delivery, cancer therapy, separations, and catalysis (Kim et al., J. Magn. Magn. Mater. 225:256 (2001); Niemeyer, Angew. Chem. Int. Ed. 40:4128 (2001); Neuberger et al., J. Magn. Magn. Mater. 293:483 (2005); Tartaj et al., J. Magn. Magn. Mater. 290:28 (2005); Dobson, Drug Dev. Res. 67:55 (2006)). However, one of the major obstacles is the lack of flexibility in surface modification and biocompatibility. Gold coating on magnetic particles provides an effective way to overcome such an obstacle via well-established surface chemistry to impart magnetic particles with the desired chemical or bio-medical properties (Daniel et al., Chem. Rev. 104:293 (2004); Xia et al., Adv. Mater. 12:693 (2000)). There have been reports on the synthesis of gold-coated magnetic core-shell particles by various methods, e.g. gamma ray, laser ablation, sonochemical method, layer-by-layer electrostatic deposition, chemical reduction, and micelle methods (Kinoshita et al., J. Magn. Magn. Mater. 293:106 (2005); Zhang et al., J. Phys. Chem. B 110:7122 (2006); Caruntu et al., Chem. Mater. 17:3398 (2005); Spasova et al., J. Mater. Chem. 15:2095 (2005); Stoeva et al., J. Am. Chem. Soc 127:15362 (2005); Lyon et al., Nano Letters 4:719 (2004); Mandal et al., J. Colloid Interface Sci. 286:187 (2005)). Recently reported was the synthesis of monodispersed core-shell Fe oxide-Au nanoparticles via coating pre-synthesized iron oxide nanoparticles (5-7 nm sizes) with gold shells (1-2 nm) (Wang et al., J. Phys. Chem. B 109:21593 (2005)). However, many of the magnetic core or shell dimensions have been limited to <15 nm. This limitation poses a serious barrier to magnetic applications where the size tunability, especially in larger sizes (up to ˜100 nm) with sufficient magnetization, is required.
The present invention is directed to overcoming these deficiencies in the art.